A conventional offshore wind turbine generator (hereinafter referred to as an “offshore wind turbine”) 1 employs, for example, a monopile foundation shown in FIG. 11 or a fixed foundation shown in FIG. 12.
Accordingly, when the offshore wind turbine 1 is constructed, a construction method is employed in which, for example, as shown in FIG. 13, a crane ship S used for construction has its outriggers So lowered onto the seabed to alleviate rolling of the hull due to ocean waves. Reference sign 2 in the drawing denotes a tower member (tower) of the offshore wind turbine generator 1, 3 denotes a nacelle, 4 denotes a rotor head, and 5 denotes a wind turbine blade.
On the other hand, if the construction is carried out using, for example, a crane ship without the outriggers So, the effect of rolling due to ocean waves must generally be reduced by selecting a crane ship larger than those of typical weight, depending on the size of the offshore wind turbine 1.
The above methods for constructing the offshore wind turbine 1, whose foundation is lowered onto the seabed, are means for constructing the offshore wind turbine 1 without causing relative misalignment by immobilizing the crane ship S or avoiding the effect of rolling.
Construction methods and so on for construction of the monopile or fixed offshore wind turbine 1 using the crane ship S are disclosed in, for example, Patent Documents 1 to 3 below.    Patent Document 1:    Japanese Unexamined Patent Application, Publication No. 2006-37397    Patent Document 2: WO 2007/091042 A1    Patent Document 3: U.S. Patent Application, Publication No. 2006/0120809, specification